1. Field of the Invention
One embodiment of the invention relates to a light-emitting element utilizing electroluminescence and a manufacturing method thereof. Further, one embodiment of the invention relates to a light-emitting device and a display device using the light-emitting element.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence. In a basic structure of such a light-emitting element, a layer containing a substance having a light-emitting property is interposed between a pair of electrodes. By applying a voltage to this element, light emission can be obtained from the substance having a light-emitting property.
Since the above light-emitting element is a self-luminous type, a display device using this light-emitting element has advantages such as high visibility, no necessity of a backlight, and the like. Further, such a light-emitting element also has advantages in that the element can be formed to be thin and lightweight and that response time is high.
Further, since a light-emitting element can be formed by stacking thin films, a display device using this light-emitting element can have a large area. That is, a planar light source can be easily provided. The feature that a large area can be provided is difficult to realize with point light sources typified by a filament lamp and an LED or with linear light sources typified by a fluorescent light. Therefore, such light-emitting elements also have a high utility value.
The light-emitting elements using electroluminescence are roughly classified in accordance with whether they include an organic compound or an inorganic compound as a substance having a light-emitting property. When an organic compound is used as a substance having a light-emitting property, the emission mechanism is as follows.
First, a voltage is applied to a light-emitting element. This allows electrons and holes to be injected from a pair of electrodes into a layer including a light-emitting organic compound. Accordingly, the light-emitting organic compound is raised to an excited state. Then, recombining carriers (electrons and holes) emit light in transition from the excited state to the ground state.
Because of the above mechanism, such a light-emitting element is called a current-excitation light-emitting element. Note that an excited state of an organic compound can be of two types: a singlet excited state and a triplet excited state, and luminescence from the singlet excited state (S*) is referred to as fluorescence, and luminescence from the triplet excited state (T*) is referred to as phosphorescence. Furthermore, it is thought that the ratio of S* to T* in a light-emitting element is statistically 1:3.
A light-emitting element using an organic compound as a substance having a light-emitting property has a lot of problems which depend on materials or element structure. In order to improve the element characteristic, development of a novel material has been carried out and improvement in an element structure has been considered.
For example, in the case where the above light-emitting element is applied to an active matrix type display device, the light-emitting element is formed over an element substrate provided with a transistor that controls light emission and the like. However, there has been a problem of decrease in aperture ratio caused by a wiring, a transistor, or the like in a structure where light emitted from a light-emitting element is extracted to the outside through the element substrate provided with a transistor and the like (bottom emission structure).
In order to solve this problem, a structure where light is extracted from the side opposite to an element substrate (a top emission structure) is proposed (see Patent Document 1, for example). By using a top emission structure, the aperture ratio can be increased and the light quantity which is extracted can be increased.